1. Field of the Invention
This invention relates generally to basket centrifuges. More particularly, this invention relates to methods and apparatus for automatically monitoring, operating, and controlling basket centrifuges using intelligent computer control systems and remote sensing devices. This invention is particularly useful for the monitoring and controlling of parameters such as feeding, cake moisture, filtration resistance (including that due to the cake, cake heel and filter media), solids volume fraction or cake porosity, wash ratio, and optimal G-force and time for the entire operating cycle.
2. Description of the Related Art
A centrifuge is a machine that uses centrifugal force for separating substances according to the difference in their physical properties. A sedimenting solid-wall centrifuge, for example, separates liquids and solids of different densities contained in a slurry mixture; a filtering "perforate-wall" centrifuge separates solids from liquids whereby the solids are retained by a filter media and the liquid is allowed to pass through. Such perforate wall centrifuges are also commonly known as "basket filtering centrifuges" or simply "basket centrifuges". Centrifugal gravity G, in units of earth's gravity g (32.2 ft/s.sup.2 or 9.8 m/s.sup.2), for basket filtering centrifuges ranges typically from 300 g to 2000 g. Examples of various basket (i.e., filtering-type batch, or perforate wall) centrifuges are disclosed in commonly assigned U.S. Pat. Nos. 5,582,742 to Wilkie et al., and 5,004,540 to Hendricks. As used herein, "basket centrifuge" refers generally to all types of perforate wall, batch filtering centrifuges, including those having solid-bottom (both base-bearing and link-suspended) and open bottom (both top-suspended and link-suspended), and top driven or bottom driven baskets.
In a basket centrifuge a feed slurry is introduced into a filtering basket rotating at a high angular velocity. After the contents have accelerated to speed, the centrifugal force results in separation of the liquid components of the slurry from the solid components, in that the liquid components (the filtrate) are forced through a filter medium supported by the perforated wall of the filtering basket while the solid components are retained on the filtering medium. The solid components remaining in the filtering basket are referred to as a cake.
With reference to FIG. 1 A, one cycle for batch filtering centrifuges comprises acceleration of the basket to intermediate (loading) speed, typically 40%-60% of full speed; loading, that is, introduction of the feed or input stream into the basket; acceleration to full speed; washing of the filter cake; drying of the filter cake; deceleration; and discharge or unloading of the filter cake. In certain cases, the wash liquid is introduced immediately after feeding before the basket is accelerated to full speed. Cycle time generally varies from several minutes to half an hour. In some pharmaceutical and specialty chemical processes, the cycle time can be as long as several hours due to the slow drainage or dewatering of liquid from the cake, in which cases the throughput is significantly reduced.
Acceleration and deceleration times depend on the moment of inertia of the basket and its total contents, and driving and braking torques. Wash times vary based on the mass of the cake, the wash ratio (the amount of wash liquid vs. the amount of residual mother liquor which it is displacing), the impurity level, and the cake resistance/permeability.
Feeding times, typically several minutes, depend on the filtration rate, which in turn depends on the cake thickness and permeability. The filtration flux is generally between 0.5 and 2 gpm per square foot of filter medium. For slow-filtering materials with low cake permeability (high cake resistance) feeding is in batches (or intermittent) to allow the filtration to "catch-up". Otherwise, the feed slurry might overflow the end weir. Dewatering times are a function of operating conditions (G and cake height) and cake properties (final cake moisture, permeability and liquid viscosity), while unloading times depend on the amount of the filter cake and its rheology. Each of the above steps may be initiated manually by an operator, or semi-automatically using programmed steps in conjunction with reset timers, speed sensors, limit switches, and the like. Usually feeding time (filtration limited) and/or dewatering time (dewatering limited) dictate the length of the cycle.
Controlling and optimizing the operation of such centrifuges is a difficult task considering the high rotational speeds of the basket, and the changing characteristics of the input or feed slurry due to upstream "upset" from crystallizer or reactor, and the filtrate and cake outputs. Also, a basket centrifuge is typically used to process different products at various times, and depending on their characteristics the products have different filtration and dewatering requirements. For some plants, the operators have been instructed to run different cycle times for various products based on the histories of each product. Some require a cycle time of only half an hour, while others can take up to eight hours. In some pharmaceutical applications, given the high value of the product, an operator needs to monitor the centrifuge until the last drop of filtrate drains out of the basket. This manual attendance becomes a time-consuming nuisance. A limited practice for control has been adopted based on products with various cycle times from past experience. Given the variability of the feed, especially due to upsets from upstream crystallizers and reactors as mentioned above, the product may not achieve the final cake dryness based on a nominal dewatering time. In these cases, the operator has to monitor and fine-tune the process for each product, which often varies from batch to batch. Otherwise the operator has to use the most conservative (worst)case when the cycle time is the longest. This unnecessarily reduces the overall throughput to the centrifuge.
However, none of the prior art is apparently directed to comprehensive, computerized control systems for operating, controlling, and monitoring basket centrifuges where manual attendance is eliminated and where the basket centrifuge is constantly optimized. The ability to provide precise, real-time control and monitoring of such centrifuges constitutes an on-going and critical industrial need, especially so that the upset or off-optimum products from the centrifuge, such as wetter cake, are not passed to the downstream dryer or recrystallizer.